Steven Brown

Research Chemist

Dr. Brown received a Ph. D. in chemical physics with Professor Fleming Crim at the University of Wisconsin-Madison. He came to NOAA in 1997 as an NRC post-doctoral fellow working with Dr. A. R. Ravishankara, was a Research Scientist with CIRES at the University of Colorado from 2000 - 2005, and has served as a federal Research Chemist since then. His major research theme at NOAA has been the chemistry and impacts of nitrogen oxides in the Earth's atmosphere. His initial research was on laboratory studies of stratospheric nitrogen oxide kinetics; more recently his focus has been on field measurements of tropospheric nitrogen oxides, particularly those that occur in the dark ("nighttime chemistry"). His other main research interest has been the development of high sensitivity optical instrumentation for laboratory and field studies of atmospheric trace gases and aerosols.

Education

Research

Atmospheric nitrogen oxides. Nitrogen oxides (NOx = NO + NO2) have both natural and anthropogenic sources, although in the troposphere they are primarily a pollutant derived from fossil fuel combustion. They regulate the abundance of ozone in all regions of the atmosphere and as such are important to air quality, climate and the stratospheric ozone layer. Laboratory and field measurements quantify their ambient concentrations and characterize the processes that govern their atmospheric chemistry.

Nighttime tropospheric chemistry. Atmospheric chemical transformation is driven by sunlight, which acts as a photolytic source of radicals to initiate chemical reactions. In the dark, however, a different set of chemical cycles involving species that are unstable in sunlight can become important. For example, the nocturnal nitrogen oxides, NO3 and N2O5, regulate the nocturnal lifetime of both NOx and ozone, initiate the oxidation of reactive VOC such as biogenic hydrocarbons, release active gas-phase halogen compounds from sea salt, and participate in heterogeneous chemistry and formation of secondary aerosol.

High sensitivity optical instrumentation. Direct absorption spectroscopy is an absolute technique for measurement of atmospheric trace gases and aerosol extinction that has been traditionally limited in atmospheric science applications because of its low sensitivity. Recent advances in optical cavities (e.g., two mirrors aligned such that light makes multiple reflections between them) have greatly enhanced the sensitivity and utility of direct absorption methods. Multiple field and laboratory instruments based on cavity ring-down and cavity enhanced absorption spectroscopies are currently in use and / or under development at ESRL / CSD.

Current Topics

Heterogeneous nitrogen oxide chemistry & halogen activation. Gas-particle reactions of nitrogen oxides are key to the regulation of atmospheric oxidant burdens, but they remain rather poorly understood. For example, recent NOAA led field work led to the discovery of nitryl chloride (ClNO2), a major source of chlorine to the atmosphere, produced by heterogeneous nitrogen oxide chemistry. We have developed key instrumentation and led major field intensives in New England, Texas and the Gulf Coast, New York City, California and Colorado to investigate these issues.

Nocturnal biogenic VOC oxidation. Biogenic VOC from terrestrial vegetation (e.g., isoprene, monoterpenes) and biogenic marine sulfur compounds (e.g., dimethyl sulfide, DMS) undergo rapid nocturnal degradation in the presence of the nitrate radical, NO3. Since NO3 is derived from NOx, an anthropogenic pollutant, these oxidative processes represent an anthropogenic perturbation of a biogenic atmospheric input. These perturbations can have important consequences, such as the formation of organic and sulfate aerosol that affect Earth's climate. These processes have been studied through recent field studies from ships (New England 2002), aircraft (New England 2004, Texas 2006), at the SAPHIR chamber in Jülich, and at the 2011 BEACHON-RoMBAS campaign at Manitou Forest in Colorado. Recent experiments include aircraft and ground based measurements in the southeast U.S. during the SENEX and SOAS campaigns.

Oil and gas emissions, wintertime ozone. The environmental impacts of the recent increase in production of oil and natural gas in North America are an important current issue. Shale gas basins in both Utah and Wyoming have recently experienced ozone air quality episodes, but only during the winter season. The mechanism for this ozone formation remains unclear, but may be related in part to oxidant formation through heterogeneous nitrogen oxide reactions. Recent field campaigns in both Utah and Colorado's front range have investigated this phenomenon along with emissions of NOx and VOCs from oil and gas activities.

Atmospheric Chemistry of Glyoxal and Nitrous Acid. Glyoxal (CHOCHO) is a reactive intermediate in the degradation of both biogenic and anthropogenic VOCs, and has been proposed as an efficient route to formation of secondary organic aerosol, SOA. Nitrous acid (HONO) forms from heterogeneous uptake of NO2, and its photolysis is thought to be an important source of OH radicals in polluted environments. Both compounds can be measured by recently developed, cavity enhanced spectrometers based on light emitting diode (LED) light sources. The new ACES (Airborne Cavity Enhanced Spectrometer) instrument provides high precision, rapid response measurements of CHOCHO, HONO and (NO2). The prototype was deployed for ground based measurements in California and Utah, and the first aircraft instrument operated successfully on NOAA P-3 aircraft during the 2013 SENEX campaign in the Southeast U.S.

Ultraviolet Broadband Aerosol Extinction. Aerosols both scatter and absorb incoming solar radiation, influencing Earth's climate. Recent developments of visible cavity ring-down and photoacoustic instruments have enabled new and detailed understanding of ambient aerosol optical properties. We have undertaken the development of broadband aerosol extinction (= absorption + scattering) at short wavelengths using cavity enhanced spectrometers with LED and other light sources. This instrument concept provides spectrally resolved measurements over a wavelength range where some aerosol species, such as "brown carbon", may have wavelength dependent absorption. The instrument has recently participated in measurements at the U.S. Forest Service Fire Lab (FLAME IV), at the SOAS campaign in the southeast U.S., and through a collaborative project with Dr. Yinon Rudich at the Weizmann Institute of Science in Israel.